Leta, Seyoum

Abstract [en]

In Ethiopia industrial effluents containing high contents of organic matter, nitrogen and heavy metals are discharged into inland surface waters with little or no pre-treatment. Significant pollution concerns related to these effluents include dissolved oxygen depletion, toxicity and eutrophication of the receiving waters. This has not only forced the government to formulate regulations and standards for discharge limits but also resulted in an increasing interest and development of methods and systems by which wastewater can be recycled and used sustainably. The need for technologies for environmentally friendly treatment of industrial wastes such as tannery wastewaters is therefore obvious. Biological processes are not only cost effective but also environmentally sound alternatives to the chemical treatment of tannery wastewaters.

The aim of the research presented in this thesis was to develop and optimize processes for biological nitrogen removal from tannery wastewaters and to identify the most efficient denitrifying organisms in tannery wastewaters laden with toxic substances. A pilot plant consisting of a predenitrification anoxic system, aerated nitrification compartment and a sedimentation tank (clarifier) all arranged in series was developed and installed on the premises of Addis Ababa University, Ethiopia. In spite of high influent chromium and sulphide perturbations over the successive feeding phases, the performance of the pilot plant was encouraging. The overall removal efficiency of the pilot plant over the experimental feeding phases varied between 82-98% for total nitrogen, 95-98% for COD, 96-98% for BOD5, 46-95% for ammonia nitrogen, 95-99% for sulphide and 93-99% for trivalent Chromium. Six isolates from over 1000 pure cultures were identified as the most efficient denitrifying bacteria. From both cellular fatty acid profiles and 16S rRNA gene sequencing, the six selected strains were phylogenetically identified as Brachymonas denitrificans in the β-subdivision of the Proteobacteria. All the six strains contain cd1-type nitrite reductase. The efficient isolates characterized in this study are of great value because of their excellent denitrifying properties and high tolerance to the concentrations of toxic compounds prevailing in tannery wastewaters. Bio-augmentation of the pilot plant with this bacterium showed a clear correlation between in situ denitrifying activities measured by nitrate uptake rate, population dynamics of the introduced B.denitrificans monitored by fluorescent in situ hybridization and the pilot plant performance, suggesting that the strategy of introducing this species for enhancing process performance has potential applications.

Moreover, the nitrate-reducing, sulphur-oxidizing bacteria (NR-SOB) were also found in the pilot plant in abundance with steady sulphide removal efficiency during the study period. This could provide opportunities for the application of biologically mediated simultaneous removal of sulphide and nitrogen from tannery effluents. In addition to enriching high consortia of denitrifiers in the anoxic system to attain high denitrification efficiency and also improving the overall nitrification efficiency of the system, the predenitrification-nitrification pilot process plant stimulated the activity of indigenous NR-SOB to simultaneously remove sulphide from the system. Thus, the pilot plant was found to be operationally efficient for the removal of nitrogen, organic matter and other pollutants from tannery wastewaters.

Abstract [en]

In order to identify potential microorganisms with high denitrifying capacity from tannery wastewaters, 1000 pure cultures of bacterial isolates from Modjo Tannery Pilot and Ethio-tannery wastewater treatment plants (WWTP), in Ethiopia, were investigated. Twenty-eight isolates were selected as efficient denitrifiers. These were Gram-negative rods, oxidase and catalase positive denitrifying organisms. The 28 denitrifying strains were further classified according to their biochemical fingerprints into three different phylogenetic groups (BPT1, BPT2 and BPT3) and seven singles. Isolates B79(T), B11, B12, B15, B28 and B38 belonging to the BPT3 cluster were found to be the most efficient denitrifying bacteria. All phenotypic studies, including cellular fatty acid profiles, showed that the 6 BPT3 isolates were closely related to each other. The 16S rRNA partial sequence analysis of type strain B79(T) (CCUG 45880) indicated a sequence similarity of 99% to Brachymonas denitrificans JCM9216 (D14320) in the beta-subdivision of proteobacteria. Further studies of the effects of chromium III and sulphide on the six Brachymonas denitrificans strains indicated that denitrification by the isolates were inhibited 50% at concentrations of 54 and 96 mg/l, respectively. The efficient isolates characterized in this study are of great value because of their excellent denitrifying properties and relatively high tolerance to the concentrations of toxic compounds (70 mg chromium/l and 160 mg sulphide/l) prevailing in tannery wastewaters.

Dalhammar, Gunnel

Abstract [en]

The objective of this study was to set-up a pilot plant and to evaluate its effectiveness for biological nitrogen and organic matter removal from tannery wastewater in Ethiopia. A pilot wastewater treatment plant consisting of a predenitrification-nitrification process was constructed and operated for 6 months. This was fed with a raw tannery wastewater obtained from the Modjo Tannery located 70 km south of the capital, Addis Ababa. Up to 98% total nitrogen and chemical oxygen demand, and 95% ammonium nitrogen removal efficiencies were achieved in the system. The average effluent ammonium nitrogen ranged from 8.4 mg l(-1) to 86.0 mg l(-1), whereas the average effluent for nitrate nitrogen ranged from 2.9 mg l(-1) to 4.4 mg l(-1). The average values of denitrification and nitrification rates determined by nitrate and ammonium uptake rates (NUR and AUR) were 8.0 mg NO3-N [g volatile suspended solids (VSS)](-1) h(-1) and 5.4 mg NH4-N (g VSS)(-1) h(-1), respectively, demonstrating that the treatment processes of the pilot plant were effective. Further studies of the effect of chromium III on AUR showed 50% inhibition at a concentration of 85 mg l(-1), indicating that this metal was not causing process inhibition during performance operations. Thus, the predenitrification-nitrification process was found to be efficient for simultaneous removal of nitrogen and organic substrates from tannery wastewaters.

Abstract [en]

Laboratory scale and pilot plant reactors were inoculated with an efficient denitrifier, Brachymonas denitrificans (CCUG 45880), in order to evaluate whether a bio-augmentation approach can be used to enhance biological nitrogen removal from tannery effluents. To determine the effectiveness of the introduced strain, denitrifying activity in the activated sludge was monitored by nitrate uptake rate ( NUR) measurement of NO3-N. Fluorescent in situ hybridization ( FISH) technique was used to monitor the growth of the augmented species. The laboratory scale nitrate removal efficiency with the introduced B. denitrificans (3.7 +/- 0.6 mg NO3-N gVSS(-1) h(-1)) was higher than that of the activated sludge without the addition of the bacteria (3.5 +/- 0.7 mg NO3-N gVSS(-1) h(-1)); the NUR in the pilot plant after and before the introduction of the strain was also of the magnitude of 12.0 +/- 1.4 and 10.6 +/- 1.4 mg NO3- N gVSS(-1) day(-1), respectively. In situ hybridization results revealed that the introduced denitrifying bacteria significantly facilitated the development of a dense denitrifying bacterial population in the activated sludge, which enhanced in situ denitrification activity. FISH data indicated that once introduced, B. denitrificans remained abundant throughout the experimental period. The ability to seed a bioreactor with bacterial strain capable of removing target pollutants from tannery effluents in a mixed microbial community suggests that this approach could have commercial applications.